Many audio amplifying circuits may generate an audible sound when operation state of the audio amplifying circuits transfers from play to mute or from mute to play. The main reason for this is that input currents at input nodes of the audio amplifying circuits do not match with each other, which introduces a voltage difference into the circuit. Typically, this sound is audible as a popping sound. As this popping sound is a disturbance to users, it would be desirable to eliminate or at least reduce this popping noise.
FIG. 1 shows an audio amplifying circuit incorporating popping noise countermeasure. The amplifying circuit comprises a first operational amplifier 11 and a second operational amplifier 12, which together serve as a differential amplifying circuit. A non-inverting input node 41 of the first amplifier 11 is coupled to a first terminal 51 via a first capacitor 13. A non-inverting input node 42 of the second amplifier 12 is coupled to a second terminal 52, and a second capacitor 14 is coupled between the second terminal 52 and ground. The first terminal 51 is configured to receive audio signals while the second terminal 52 is configured as an AC ground relative to the audio signals received at the first terminal 51. A first output node 45 of the first amplifier 11 is coupled to an inverting input 43 of the first amplifier 11 via a first resistor 15. A second output node 46 of the second amplifier 12 is coupled to an inverting input 44 of the second amplifier 12 via a second resistor 16. The output nodes 45 and 46 of the amplifiers 11, 12 are coupled across a load 17, typically a loudspeaker able to give out sounds according to the current flowing therethrough. A third resistor 18 and a fourth resistor 19 coupled in series are interposed between the inverting inputs 43, 44 of the first and second amplifiers 11, 12. The common node of the third resistor 18 and the fourth resistor 19 is coupled to ground via a fifth resistor 20 and a third capacitor 21 coupled in series. The series-coupled fifth resistor 20 and third capacitor 21 have the function of filtering noise and adjusting frequency response of the amplifying circuit.
The amplifying circuit further comprises a reference circuit component providing a reference voltage to the non-inverting input 41, 42 of the amplifiers 11, 12. The reference circuit component comprises a sixth resistor 22, a seventh resistor 23, an eighth resistor 24, a fourth capacitor 25, a ninth resistor 26 and a tenth resistor 27. The sixth resistor 22 and the seventh resistor 23 forming a divider are series-coupled between a supply power VCC and ground, which set the voltage at node 53 to a reference voltage. For example, the reference voltage may be chosen to be equal to VCC/2 and the values of resistor 22, 23 are then set to a same value. The common node of the resistors 22 and 23, that is, the node 53, is coupled to a first node of the eighth resistor 24, and the other node of the eighth resistor 24 serves as a reference node 54, which is coupled to ground via the fourth capacitor 25. The fourth capacitor 25 has the function of rejecting power noise from the supply power VCC. The reference node 54 is coupled to the non-inverting input node 41 of the first amplifier 11 via the ninth resistor 26, and coupled to the non-inverting input node 42 of the second amplifier 12 via the tenth resistor 27, respectively.
The amplifying circuit is such designed to reduce the popping noise. Usually, audio circuits are fit to more than one channel, for example 2 or 4 channels, in the purpose of reproducing sounds more vividly and accurately. Still referring to FIG. 1, the amplifying circuit is fit to a four-channel audio amplifier. As a result, it is required that a second input current Iin2 flowing from the input node 42 into the second capacitor 14 should be four times bigger than a first input current Iin1 flowing from the input node 41 into the first capacitor 13. As is known, when there are no signals received at the input nodes 41 and 42, there should be no voltage difference between these two nodes. Therefore, the ratio of the capacitors 13 and 14 should be four in accordance with the ratio of the input currents and Iin1 and Iin2. At the same time, in order to achieve the requirement of current matching, the ninth resistor 26 and the tenth resistor 27 that are separately coupled to the non-inverting inputs should be set in accordance with the currents flowing therethrough. That is, the value of the ninth resistor 26 should be four times bigger than that of the tenth resistor 27.
However, it is difficult to assure the first and second capacitors 13, 14 to satisfy the requirement. Mismatch between the first and second capacitors 13, 14 frequently happens. The mismatch between capacitors 13 and 14 may significantly influence the normal operation of the amplifying circuit, and the popping noise cannot be fully eliminated, especially when the supply power fluctuates greatly, for example, the output of supply power in a car changes a lot when the car is starting.
FIG. 2 shows the evolution of voltages along time at given points of the amplifying circuit of FIG. 1, when there is a big pulse from the supply power. Curve Vs shows the variation of the supply voltage along time. Curve Vin1 shows the variation of the voltage at the input node 41 along time. Curve Vin2 shows the variation of the voltage at the input node 42 along time. The settle time, that is, the rise or fall time of the voltage at the input node 41 is mainly determined by the values of the capacitor 13 and the input current Iin1, and the settle time of the voltage at the input node 42 is determined by the values of the capacitor 14 and the input current Iin2. However, the ratio of the capacitors does not always accord with the ratio of the input currents flowing into the capacitors, which leads to different settle times for input nodes 41, 42, as illustrated in FIG. 2. In FIG. 3, curve Vin1−Vin2 shows the voltage difference between voltages at the input nodes 41 and 42. Further, the voltage difference is reflected on the load 17, multiplied by the amplifying gain of the amplifying circuit. Due to a high amplifying gain, the voltage applied to the load 17 is often sufficient to cause a characteristic audible and unpleasant noise.
Moreover, the device mismatch between the first capacitor 13 and the second capacitor 14 can also introduce electromagnetic noise such as global system for mobile communication (GSM) noise into the amplifying circuit, which is amplified to a bigger noise by the subsequent amplifiers 11 and 12.
Thus, there is a need for improving the noise performance of the audio amplifying circuit.